Sounds like the battery needs to be replaced. It's probably SLA in there right now, and those dont' last long (especially if left discharged for any length of time). You can just replace them with new SLA of the same type, or you can go thru like in this thread and others and replace with Lithium of one type or another, which will give better performance and last longer, but will probably be more expensive to start with.
If it's already lithium, then the BMS in the battery has probably shut off the input to protect you from a fire due to damaged or defective cells. You would have to open the battery and measure the voltage of each cell or cell group in that case, and list them in your post, from most negative to most positive, so we can help you figure out if it is possible to repair, or worth it, or better to get a new battery.